U.S. Pat. No. 4 431 734 discloses an enzymatic process for treating xanthan gums in order to improve the filterability and the capacity of xanthan gum aqueous solutions to be injected into and circulate through oil formations, so as to enhance the crude oil recovery. This process comprises a suitable treatment with two enzyme systems, the first of polysaccharase type in acid or substantially neutral pH, the second of protease type in basic, neutral or acid pH, and provides for an improvement of the injectivity and flowing properties of xanthan solutions through the oil formations without loss of the intrinsic properties and particularly of the thickening power of the polysaccharide.
According to this patent the enzymatic treatments by means of an enzyme of polysaccharase type and of an enzyme of protease type may be performed either simultaneously at a pH compatible with a sufficient activity of both enzymes or successively at a pH suitable for the type of enzyme selected in each step. The best results are obtained with an operation in two successive steps, the first step being performed with polysaccharase and then the second step with protease.
The enzyme extracts called polysaccharases, usually obtained by aerobic cultivation of fungi pertaining to the category of Basidiomycetes or fungi pertaining for example to the Aspergillus, Fusarium, Myrothecium, Penicillium, Polyporus, Rhizopus, Sclerotinia, Sporotrichum and Trichoderma genera can be used in the process of this patent.
These enzymes, liable to hydrolyze polysaccharides, are usually sold in the trade under the name of cellulases and are used in the process according to this patent under pH, temperature and salt concentration conditions such that the characteristics of the xanthan gum itself is not substantially modified.
The second category of enzyme extracts to be used complementarily to the preceding category in the process according to this patent is formed by the category of bacterial proteases. These proteases are generally produced by microorganisms of the Bacillus genus such as B. Subtilis, B. licheniformis, B. amyloliquefaciens and B. pumilis, or still of Streptomyces genus such as S. fradiae, S. griseus and S. rectis. The enzyme source is however not critical. These proteases are called acid, neutral or alkaline proteases when their acitivity is optimum respectively at slightly acid, neutral or basic pH values.
The enzymatic treatment according to this process is preferably performed during a total incubation period of 0.5-60 hours, preferably 4-48 hours, at temperatures ranging from room temperature (about 25.degree. C.) up to about 65.degree. C., preferably at 40.degree.-60.degree. C. Short treatment periods are preferably associated with high temperatures and conversely.
The enzymatic treatments are performed in aqueous medium wherein the alkali and/or alkaline-earth metals are dissolved in a proportion of at least 10.sup.-2 equivalent/liter, preferably at least 10.sup.-1 equivalent/liter. The relative synergism effect obtained by this process is however the more substantial as the salt content of the treatment water is higher. A particular aspect of the process according to this patent consists in the fact that the synergism activity of the two enzyme preparations is also obtained in the presence of divalent ions, e.g. Ca.sup.++ or Mg.sup.++, particularly the field water.